https://blog.gongquiz.com/2021/01/23/%eb%8b%a4%ec%9d%8c-%ec%a4%91-%ec%9d%91%eb%a0%a5-%eb%aa%a8%ec%96%b4%ec%9b%90stress-mohr-circle%ec%97%90-%eb%8c%80%ed%95%9c-%ec%84%a4%eb%aa%85%ec%9c%bc%eb%a1%9c-%ec%98%b3%ec%9d%80-%ea%b2%83%ec%9d%80/